There have widely been known photopolymerizable inkjet inks containing (meth)acrylic acid esters, and photopolymerizable inkjet inks containing (meth)acrylic acid esters and vinyl ethers in combination (see, for example, PTL 1).
However, many of the monomers used in such conventional photopolymerizable inkjet inks have toxicity. Although (meth)acrylic acid esters are inexpensive and easily procurable, most of them exhibit high skin sensitization, which is a property of causing allergy on the skin brought into contact with them. Conventional art has not shown any solution to this problem.
The present inventors previously conducted studies on this problem and found some (meth)acrylic acid esters and (meth)acrylamides negative for skin sensitization. However, in an attempt to produce an ink with a practical level of curing property from these compounds, the resultant ink disadvantageously becomes more viscous than commonly used inkjet inks. Thus, an ejection head able to be heated to a high temperature sufficient to reduce the viscosity of the ink must be used to inkjet the ink. In addition, the inner pressure of the head has to be considerably increased to eject the ink, making it hard to eject the ink stably.